Pain is the major symptom in osteoarthritis (OA), contributing to impaired function and loss of quality of life, and one of the leading causes of impaired mobility in the elderly population in the US. Our lack of understanding the mechanisms underlying chronic pain in general, and chronic pain associated with OA in particular, accounts for the general ineffectiveness of currently available treatment options. Relief from severe OA pain remains an unmet medical need and a major reason for seeking surgical intervention. In spite of its major impact on quality of life and health care management, our understanding of the mechanisms of pain in human OA remains very poor. The long-term goal of this proposal is to define origins and mechanisms of pain in OA, thus enabling identification of new targets, and development of new therapies and biomarkers for OA pain. Compelling motivating data, along with novel genetically modified mice, have created a unique opportunity for immediate studies and rapid advances in this highly understudied area. The goal of these studies is three-fold: 1) Use a validated murine OA model, destabilization of the medial meniscus (DMM), to quantitatively measure pain and determine concurrent cellular and structural alterations in joint tissues and their sensory innervation (peripheral component and dorsal root ganglia). The goal is to make a qualitative and quantitative assessment of pain in the 16-week course of the disease using techniques that evaluate mechanical allodynia, spontaneous pain behavior and gait. A detailed correlation of pain measures will be made with pathological changes in all joint tissues (cartilage, bone, synovium, and meniscus) and with concomitant changes in the number and location of afferent neurites in the joint and dorsal root ganglia; 2) Determine the effect of inhibiting ADAMTS-5 on pain detected in association with structural joint and neuronal changes in the DMM model. Inhibition will be achieved in vivo a) by using Adamts5 null mice and b) by pharmacological inhibition of ADAMTS-5 with potent and selective inhibitors. The absence of active ADAMTS-5 protects against OA progression in the DMM model. ADAMTS-5 inhibitors are being developed as disease-modifying OA drugs, without clear understanding of their potential to affect pain. These studies should allow us to determine whether these inhibitors will also affect pain, and how long after onset of disease the pain associated with OA structural changes is reversible or at what stage the pain has become irreversible. Findings here are expected to have important implications for pain management in human OA; 3) Dissect molecular pathways participating in onset and chronicity of OA-related pain in the DMM model, through temporal analysis of molecular markers of nociceptive pathways, glial activation, and hyalectan fragments. Specifically, the temporal role of the NGF-mediated and the MCP-1-mediated nociceptor pathway will be explored. Genes implicated in these animal studies will provide the basis for genetic linkage analysis in large human OA cohorts with known pain and disability measures such as the WOMAC score.